1. Field of the Invention
The present invention relates to a fluorescent lamp operating apparatus capable of improving a lumen maintenance factor by suppressing the reaction between mercury and both of a phosphor and a glass tube, and thus extending life of a fluorescent lamp. The present invention also relates to a fluorescent lamp operating apparatus capable of improving the lumen maintenance factor and thus enhancing the start-up characteristic of a fluorescent lamp.
2. Description of the Related Art
FIG. 10 is a schematic view of a conventional fluorescent lamp operating apparatus 1000.
The fluorescent lamp operating apparatus 1000 in FIG. 10 includes a fluorescent lamp 101 and a ballast circuit 102 for starting the fluorescent lamp 101. In this specification, the expression "starting the lamp" means to start the lightening of the lamp (i.e., to ignite the lamp).
The ballast circuit 102 includes a DC power supply circuit 103, an inverter circuit 104, a choke coil 111, and a capacitor 112. The DC power supply circuit 103 rectifies and smooths a commercial AC power supply 105 by a rectifier 106 and a capacitor 107 so as to convert an AC current to a DC current.
The inverter circuit 104 includes transistors 108 and 109, a capacitor 110, and a driving circuit 113. The transistors 108 and 109 are turned ON or OFF alternately by a signal from the driving circuit 113. When the transistor 108 is ON and the transistor 109 is OFF, a current flows in a path beginning at the capacitor 107 and continues in the order of the transistor 108, the capacitor 110, the choke coil 111, the fluorescent lamp 101, and the capacitor 107. When the transistor 108 is OFF and the transistor 109 is ON, a current flows in a path beginning at the capacitor 110 and continues in the order of the transistor 109, the fluorescent lamp 101, the choke coil 111, and the capacitor 110. Thus, an output from the DC power supply circuit 103 is converted into an AC current of several tens of kilohertz.
The fluorescent lamp 101 has a structure shown in FIGS. 11A and 11B. FIG. 11A is a cross-sectional view of the fluorescent lamp 101 along a longitudinal axis thereof, and FIG. 11B is a cross-sectional view of the fluorescent lamp 101 in a radial direction thereof.
Specifically, the fluorescent lamp 101 has the following structure. Electrodes 116 and 117 are respectively sealed at both ends of a glass tube 114 coated with a phosphor 115. In the glass tube 114, mercury and filling gas, such as argon and the like, for lowering the breakdown voltage at the start of operation by the Penning effect are sealed.
The fluorescent lamp operating apparatus 1000 having the above-described structure operates in, for example, the following manner.
When the power supply is turned ON, a preheating current flows in a path beginning at the inverter circuit 104 and continues in the order of the choke coil 111, the electrode 116, the capacitor 112, the electrode 117 and the inverter circuit 104. Moreover, a high voltage is applied to the fluorescent lamp 101 by resonance of both of the capacitors 110 and 112 and the choke coil 111. When the electrodes 116 and 117 are sufficiently heated by the preheating current, breakdown occurs and discharge plasma is formed in the glass tube 114 so as to start discharge. Due to the discharge, the mercury sealed in the glass tube 114 radiates ultraviolet rays mainly having a wavelength of 254 nm, and the ultraviolet rays excite the phosphor 115 applied on the glass tube 114, thereby causing the phosphor 115 to generate visible light. After this, the fluorescent lamp 101 maintains its operation by the AC power supplied from the inverter 104.
In such a fluorescent lamp 101, the lumen flux generally is reduced over the period in which the fluorescent lamp is operated. There are various causes for such a phenomenon. Main causes include (1) the reaction between the phosphor 115 and mercury ions, i.e., deterioration of the phosphor 115 by the mercury ions being adsorbed to the phosphor 115; and (2) reduction in the radiation amount of ultraviolet rays having a wavelength of 254 nm which is caused by the amount of mercury in the discharge space being reduced as a result of the reaction between the glass tube 114 and the mercury ions which form the fluorescent lamp 101.
As a solution of this problem, Japanese Laid-Open Publication No. 62-229752 discloses a method of forming a metal oxide protective film on a phosphor layer which is applied on the glass tube and thus suppressing the reaction between mercury and both of the phosphor and the glass tube. Another method of providing a protective film formed of alumina or the like between the glass tube and the phosphor layer and thus preventing the reaction between mercury and the glass tube has been proposed.
The fluorescent lamps obtained in these manners require a high voltage to be applied thereto at the start of operation. Accordingly, it has been proposed to provide the glass tube with a conductive film as a start assistance device. A fluorescent lamp having such a structure is referred to as a rapid-start fluorescent lamp.
The above-described fluorescent lamps have the following problems.
For example, in the case of the conventional fluorescent lamp 1000 described above, when the transistor 108 is ON and the transistor 109 is OFF, the current flows in the order of the DC power supply circuit 103, the transistor 108, the capacitor 110, the choke coil 111, the fluorescent lamp 101 and the DC power supply circuit 103. Since the output from the DC power supply circuit 103 is positive, the potential of the discharge plasma generated in the glass tube 114 is positive. Therefore, an electric field is generated in the glass tube 114 in a direction toward an infinite ground potential point (in a direction shown by the arrows in FIG. 11, i.e., from the center of the glass tube 114 toward the inner surface of the glass tube 114). The mercury ions, which are positive ions, receive the force toward the inner surface of the glass tube 114 by the electric field. As a result, as shown in FIG. 12, the mercury ion density is lower at the center of the glass tube 114 than in the vicinity of the inner surface thereof. Accordingly, the reaction between the mercury ions and both of the phosphor and the glass tube is accelerated, thereby reducing the amount of mercury contributable to light emission. In consequence, the life of the fluorescent lamp 101 is adversely effected.
The method disclosed in Japanese Laid-Open Publication No. 62-229752 cannot solve this problem for the following reason. It is difficult to apply a protective film having a uniform thickness on the phosphor, and therefore the mercury ions react with the phosphor and the glass tube in a portion of the protective film having a lesser thickness. Thus, the reaction cannot be suppressed sufficiently.
The rapid-start fluorescent lamp improves the start-up characteristic by lowering the breakdown voltage through formation of a conductive film, but is not effective on extension of life of the fluorescent lamp.